1. Field of the Invention
The present invention relates to a thermal infrared image sensor, in which the infrared-rays are detected as heat by detectors arranged in multiple pieces.
2. Description of the Related Art
In a thermal infrared image sensor sensing infrared-rays as heat, a plurality of infrared detectors serving as pixels are arranged in two dimensions. As an example of the thermal infrared image sensor, a configuration arranging a plurality of detector cells, each of which is implemented by a silicon p-n junction diode is proposed (cf. Japanese Patent Laid-Open No. 2001-281051).
The thermal infrared image sensor detects variations of the diffusion potential or the number of generated carriers, in which the temperature of p-n junction diode changes by infrared radiations irradiated from object body. The temperatures of respective infrared absorption bodies, which are disposed in each of the pixels, varies by the irradiation of infrared-rays in the thermal infrared image sensor, and the band structure of the p-n junction varies because the junction temperature of the p-n junction diode is changed. The irradiated intensity of infrared-rays can be measured, for example, by observing the voltage variation of the bit lines, and the surface temperature of the object can be detected. In a thermal infrared image sensor disclosed in Japanese Patent Laid-Open No. 2001-281051, plural cavities are arranged in a matrix-form at the surface of the substrate. Supporting beams are attached to each of the cavities so that the detector portion of each pixel can be supported in a hollow state. The detector portion is maintained in a quasi-thermal isolation state by a configuration in which the detector portion is bridged over the hollow state. The detector portion encompasses an infrared absorption layer, which converts an incident infrared ray into thermal energy by absorbing the infrared ray, and a thermoelectricity-converting portion, which converts the temperature change in the infrared absorption layer to an electrical signal. Horizontal address lines and vertical signal lines are connected to the thermoelectricity-converting portion of each pixel.
The horizontal address lines and the vertical signal lines are arranged in a grid shape such that the horizontal address lines and the vertical signal lines intersect perpendicularly each other. A pulse signal is applied to the horizontal address line in a signal read-out period. Because the detector portion is bridged above the hollow cavity at the surface of the substrate with the supporting beams so as to form the thermal insulation structure, it is hard to transport the heat outwardly, which is generated in the detector portion. In the earlier thermal infrared image sensors, the bias current value must be set such that the signal-to-noise ratio becomes large in order to raise sensitivity. In addition, the temperature sensitivity detecting the infrared-rays becomes larger as the thermal confinement effect becomes larger. As a result, “a self-heating phenomenon” is generated, in which the temperature of every detector portions rises as shown in FIG. 1B. The self-heating phenomenon is apt be generated when a bias pulse is applied to the detector portion so as to cause breakdown phenomenon. The breakdown phenomenon of the detector portion ascribable to the thermal runaway is not generated in a blanking period, because the temperature of the detector portion is low.
A nighttime front monitoring of vehicles using the thermal infrared image sensor has being tried recently, for detecting the information not provided by normal visible camera. For mounting the thermal infrared image sensor on vehicles, a high frame rate (FR) of several times of the present thermal type infrared camera is required.
However, in an earlier thermal infrared image sensor, when the frame rate rises, the thermal energy accumulated at the detector portion due to the self-heating phenomenon, cannot be dissipated sufficiently in the blanking period as shown in FIG. 1B, and as the thermal energy at the detector portion is accumulated further and further, the thermal breakdown phenomenon is caused. In order to prevent the thermal breakdown phenomenon, it is necessary to decrease the thermal resistance of the supporting beam or to decrease the heat capacity of the detector portion so as to reduce the thermal time constant. However, if the thermal time constant decreases, the sensitivity of the thermal infrared image sensor decreases simultaneously.
In view of these situations, it is an object of the present invention to provide an infrared detector and an infrared image sensor using the infrared detector as a pixel, in which the thermal breakdown due to the self-heating phenomenon is not generated, without decreasing the sensitivity